On Fri, 30 Jul 2010 12:24:43 +0100,
Chris Tolley wrote:

The thinking distance is merely the speed in mph expressed in feet. The
stopping distance is merely the speed in mph squared and divided by 20,
then expressed in feet.

vmph v ft thinking + v*v/20 stopping

20mph --- 20 ft thinking + 20*20/20 stopping ---- 40ft
30mph --- 30 ft thinking + 30*30/20 stopping ---- 75ft
40mph --- 40 ft thinking + 40*40/20 stopping ---- 120ft
..
70mph 70 70*70/20 ---- 315ft


Why is this surprising? This is just thinking time = 2/3 second and
CoF=2/3 and then rounded to easy numbers.

If you wanted it in mph - metres the formula would be

x mph - 3x/10 thinking distance and 3x^2/200 stopping distance

20mph - 6m thinking and 6m stopping - 12m overall
30mph - 9m thinking and 13.5m stopping - 22.5m overall
40mph - 12m thinking and 24m stopping - 36m overall
....
70mph - 21m thinking and 73.5m stopping - 94.5m overall


If you want it in kph - metres the formula would be

x kph - x/5 thinking distance and 6*x^2/1000

20kph - 4m thinking and 2.4m stopping
30kph - 6m thinking and 5.4m stopping
40kph - 8m thinking and 9.6m stopping
....
110kph - 22m thinking and 72.6m stopping



As for your Ford Anglia allegation, the Highway Code predates Ford
Anglias by several decades. The same figures were included in the 1946
HC, and may have been in versions before that; I can't be bothered to
look them up.

But you only need to pick a reasonable value for thinking time and CoF
and the rest is basic physics.

Thinking times from about .7s to about 1.5s is reasonable depending on
what you really mean by "alert driver" and CoF between about .5 and .8
for rubber on dry asphalt for a typical road-legal car at road-legal
speeds.

Obviously, this breaks down for high performance cars at very high
speeds. I doubt that any road-legal car generates signficant down force
at speeds much below about 100mph due to the dire effect it has on fuel
consumption.

Tim.


--
God said, "div D = rho, div B = 0, curl E = - @B/@t, curl H = J + @D/@t,"
and there was light.

http://www.woodall.me.uk/

"Tim Woodall" wrote in message
e.uk

Obviously, this breaks down for high performance cars at very high
speeds. I doubt that any road-legal car generates signficant down
force at speeds much below about 100mph due to the dire effect it has
on fuel consumption.

What about the sports cars which have retractable rear spoilers that pop
up at, say, ~75mph?

On Sat, 31 Jul 2010 19:55:31 +0100,
Recliner wrote:
"Tim Woodall" wrote in message
e.uk

Obviously, this breaks down for high performance cars at very high
speeds. I doubt that any road-legal car generates signficant down
force at speeds much below about 100mph due to the dire effect it has
on fuel consumption.

What about the sports cars which have retractable rear spoilers that pop
up at, say, ~75mph?


No idea. But if they do generate appreciable down force then they'll
also cause significant drag and you'll see that in the fuel economy.

Tim.

--
God said, "div D = rho, div B = 0, curl E = - @B/@t, curl H = J + @D/@t,"
and there was light.

http://www.woodall.me.uk/

"Tim Woodall" wrote in message
e.uk
On Sat, 31 Jul 2010 19:55:31 +0100,
Recliner wrote:
"Tim Woodall" wrote in message
e.uk

Obviously, this breaks down for high performance cars at very high
speeds. I doubt that any road-legal car generates signficant down
force at speeds much below about 100mph due to the dire effect it
has on fuel consumption.

What about the sports cars which have retractable rear spoilers that
pop up at, say, ~75mph?


No idea. But if they do generate appreciable down force then they'll
also cause significant drag and you'll see that in the fuel economy.

Yup, but that's probably not something that owners of cars with
automatic rear spoilers (eg, Porsches) worry about too much.

On Sat, 31 Jul 2010 19:55:31 +0100, "Recliner"
wrote:
"Tim Woodall" wrote in message
. me.uk

Obviously, this breaks down for high performance cars at very high
speeds. I doubt that any road-legal car generates signficant down
force at speeds much below about 100mph due to the dire effect it has
on fuel consumption.

What about the sports cars which have retractable rear spoilers that pop
up at, say, ~75mph?


Some of them pop up at much lower speeds - I think the Porsche 911 and
Audi TT spoilers pop up at 40 mph.

"Bruce" wrote in message

On Sat, 31 Jul 2010 19:55:31 +0100, "Recliner"
wrote:
"Tim Woodall" wrote in message
e.uk

Obviously, this breaks down for high performance cars at very high
speeds. I doubt that any road-legal car generates signficant down
force at speeds much below about 100mph due to the dire effect it
has on fuel consumption.

What about the sports cars which have retractable rear spoilers that
pop up at, say, ~75mph?


Some of them pop up at much lower speeds - I think the Porsche 911 and
Audi TT spoilers pop up at 40 mph.

I think it's 75 mph by default with both of those cars, but may be lower
in other cars. It can also come up at lower speeds to cool a hot engine
in the case of the Porsche. Of course, they can usually be deployed
manually, which is especially useful in countries with speed limits 75
mph, like the UK!

In message , Bruce
writes
Some of them pop up at much lower speeds - I think the Porsche 911 and
Audi TT spoilers pop up at 40 mph.
I remember reading about this. In Germany, they deploy at the proper
speed of 90 mph, but in the UK where the speed limit is 70 mph they come
up at 50 mph to allow the owner to show off, it's a sales gimmick.
--
Clive

On Sat, 31 Jul 2010 19:55:31 +0100, Recliner wrote
"Tim Woodall" wrote in message
e.uk

Obviously, this breaks down for high performance cars at very high
speeds. I doubt that any road-legal car generates signficant down
force at speeds much below about 100mph due to the dire effect it has
on fuel consumption.

What about the sports cars which have retractable rear spoilers that pop
up at, say, ~75mph?

In many cases they have an appreciable effect at speeds not often reached in
the UK.

In order to give them some 'pose value' they are recalibrated to rise at
speeds more likely to be achieved in the UK, otherwise the driver might not
feel they're getting value for money/the required pose effect.

Of course, there is an argument that says a properly designed car shouldn't
need 'add-on' wings/spoilers etc fixed or otherwise.

On Sun, 1 Aug 2010 15:01:31 +0100, Stimpy
wrote:

Of course, there is an argument that says a properly designed car shouldn't
need 'add-on' wings/spoilers etc fixed or otherwise.


Many cars suffer from increased lift at higher speeds. The spoiler
provides increased downforce to counter this.

The Audi TT (first model) suffered a significant increase in rear lift
at higher speeds. In some markets, but not the UK, the automatic rear
spoiler was standard equipment. In the UK, which has a 70 mph limit,
it was an optional extra, presumably on the basis that it was not
actually needed at 70 mph.

There was some discussion in the motoring press about whether TTs sold
in the UK should all be so fitted, in case they were taken to (for
example) Germany and driven at much higher speeds - some stretches of
German autobahn have no speed limit for cars in good weather.

IIRC Audi UK accepted the suggestion. Subsequently all TTs sold in
the UK had the spoiler fitted as standard.

Bruce gurgled happily, sounding much like they
were saying:

Many cars suffer from increased lift at higher speeds. The spoiler
provides increased downforce to counter this.

The Audi TT (first model) suffered a significant increase in rear lift
at higher speeds. In some markets, but not the UK, the automatic rear
spoiler was standard equipment. In the UK, which has a 70 mph limit, it
was an optional extra, presumably on the basis that it was not actually
needed at 70 mph.

There was some discussion in the motoring press about whether TTs sold
in the UK should all be so fitted, in case they were taken to (for
example) Germany and driven at much higher speeds - some stretches of
German autobahn have no speed limit for cars in good weather.

IIRC Audi UK accepted the suggestion. Subsequently all TTs sold in the
UK had the spoiler fitted as standard.

Not an automatic rear spoiler, but the little permanently fixed one.
Also, ESP stability control. Both were also retro-fitted in a recall.

Have a google for "vosa R/2000/008".